Metal-air batteries including zinc-air batteries offer the advantage of lightweight and very high energy densities (up to 300 WHr/Kg) over known conventional batteries such as the lead-acid type employed in automotive vehicles. Zinc-air batteries can be manufactured on a commercial production basis at low cost and with a high degree of safety. However, commercial applications of zinc-air batteries have previously been limited to primary or non-rechargeable type of batteries. Experimental rechargeable zinc-air batteries have been built for use in automotive applications. These batteries use an excess of liquid electrolyte stored in a reservoir. They usually include a pump to recirculate the electrolyte. Such systems are impractical for miniature consumer applications ranging from radios to portable computers because of their mechanical complexity and lack of leak resistance. If the battery is cut, electrolytes will be discharged from the reservoir.
A zinc-air battery generally includes a porous zinc anode, an air cathode formed of a carbon-containing membrane, and a porous material containing a liquid electrolyte which is sandwiched between the anode and cathode. A major problem which has existed in the development of rechargeable zinc-air batteries is that oxygen gas which is generated by the anode during the charging cycle, forces electrolyte accumulated above and anode upwardly into and through the air cathode. This results in loss of electrolyte and contamination of electrolyte through reaction with the atmosphere and catalyst in the air cathode. Water in the electrolyte evaporates through the porous cathode requiring provision for excess electrolyte stored in reservoirs.
These leaking problems are avoided by use of an anode wrapped in a porous separator cloth as disclosed in U.S. Pat. No. 4,957,826, the disclosure of which is expressly incorporated herein by reference. All the electrolyte is contained in the separator cloth. There is no electrolyte reservoir. The construction of the cell is simplified and the cell does not leak even when cut in half. The battery can be used in any orientation.
Another major problem with current metal-air batteries is the tendency of the air cathode to separate from the electrolyte material over the lifetime of the battery, resulting in the formation of air pockets between the air cathode and the electrolyte. As the metal anode reacts with the electrolyte during battery operation, a metal oxide or a metal hydroxide is formed. The metal oxide or metal hydroxide has a much larger volume than the metal electrode. The increased volume places pressure on the internal walls of the battery container resulting in an expansion of the container itself. This expansion in turn results in a separation of the air cathode membrane from the electrolyte, and the creation of the air pockets between the air cathode and the electrolyte. The air pockets substantially interfere with the battery output, and result in premature degradation of the battery.
The use of a battery cell case as disclosed in U.S. Pat. No. 4,894,295 prevents leakage or separation of the layers of the cell. The cell case has a concave bottom in contact with the anode. As the metal anode is oxidized and expands during discharge, the flexible case flexes to accommodate expansion.
The flexing of the bottom wall also applies pressure to the porous-electrolyte laden sheet enveloping the anode which pumps the liquid electrolyte upwardly into the upper layer of the porous sheet adjacent the cathode as disclosed in U.S. Pat. No. 4,913,983. This action of the flexible battery case eliminates the need for complex mechanical pumping of electrolyte within the cell.
The large energy to weight advantage of metal/air batteries is due to the absence of internal oxidized cathode material within the cell. The cathodic action is provided by air depolarization by the porous cathode. The air breathing nature of these cathodes exposes the internal components of the battery to the air environment. Thus, water vapor and carbon dioxide can freely pass through the cathode into the electrolyte. The cell will gain or lose water depending on the humidity and temperature of the ambient air and the battery and the hygroscopic nature of the electrolyte. Furthermore, basic electrolytes such as metal hydroxides which are typically used in zinc-air batteries, will react with atmospheric CO.sub.2. The acidic carbonic acid formed on dissolving CO.sub.2 in the electrolyte will eventually neutralize the basic electrolyte. This drastically reduces the conductivity of the electrolyte and reduces the output and limits the useful lifetime of the battery.
Transport of water vapor and CO.sub.2 into the cell through the porous cathode is even more detrimental to a secondary or rechargeable battery. The bifunctional cathode utilized in a secondary battery must be capable of expelling oxygen to the exterior of the cell during recharge. The cathode must maintain the ability to breathe easily in both directions even after many cycles of charge and discharge.
The typical cathode used in secondary metal-air battery utilizes a layer of porous carbon-filled hydrophobic polymer deposited on a porous substrate such as a screen. A catalyst layer is disposed on the other sided of the screen. The carbon pore structure degrades and becomes more porous as the cathode is subjected to repeated alternating cycles of oxidation and reduction during discharge and recharge cycles. This leads to increased air flow and may eventually lead to electrolyte weepage as the pores becomes large enough to allow liquid flow.
The effects caused by degradation of the cathode are minimized by utilizing an air chamber above the cathode through which air flows across the cathode and by the use of selectively closable ports to the chamber to form an air-tight enclosure which isolates the cell from air when not in use as disclosed in U.S. Pat. No. 4,894,295 entitled METAL-ALLOY AIR BATTERY, granted Jan. 16, 1990, the disclosure of which is expressly incorporated herein by reference.